Abstract Detail

The Geography of Follicle Pubescence in Physocarpus opulifoliuss.l. (Rosaceae): Implications for Taxonomy.

Precise taxonomic identification is a critical prerequisite for ecological, taxonomic, and systematics research, but can be complicated by cryptic morphological variation. Cryptic variation has long confounded the taxonomy of Physocarpus (Rosaceae), with conflicting circumscriptions predominant in P. opulifolius (L.) Maxim. sensu lato, in particular. Here, we assess variation for carpel and follicle pubescence, a key diagnostic character for the P. opulifoliuss.l. complex. Data were collected from nearly 600 fruiting herbarium accessions obtained from across the range of this complex in North America. We then assessed the statistical and geographic distribution of this variation. As suggested by Rydberg and others, follicle pubescence is bimodally distributed and exhibits a strong geographic signal. An eastern, glabrous (or nearly so) form occurs in temperate forests of the Appalachian, Atlantic Highlands of New England, and eastern Great Lakes Region (P. opulifoliuss.s.), while a more western, pubescent form occurs in the Interior Highlands, Driftless Area, and western Great Lakes region, with disjunct populations in the Sand Hills of Nebraska, Black Hills of South Dakota and Front Range of Colorado (P. intermedius [Rydb.] Schneid.). Putative hybrids are infrequent, but occur in the Ozark Highlands of Missouri, southern Great Lakes region, and Central Ontario. We conclude that variation for this character merits taxonomic recognition, which also has implications for downstream research (e.g., floristics, phylogenetics, ecological niche modeling).